Stackers for receiving sheet materials such as signatures or newspapers in a stream and for forming them into stacks are well known. Typical are the stackers shown in U.S. Pat. Nos. 4,139,191; 4,037,525; 3,851,773; 3,538,818; 3,479,932; 3,450,275; 3,480,503; and 2,672,079. These stackers receive a stream of publications, form them into stacks and deposit them on a conveyor for subsequent handling.
One particular type of stacker includes a platform that moves downward as the sheet materials are being stacked on the platform. Such a stacker is disclosed in a pending application of Mohanjit Singh Chandhoke and myself, Michael B. Duke, entitled "Apparatus for Forming a Stack of Signatures", U.S. Application No. 518,015, which is assigned to the assignee of this invention. Such a stacker may include a counter positioned upstream of the stacking workstation for counting the number of signatures being fed to a stack. Sensors may be utilized to determine stack dimensions. When a predetermined number of signatures needed to complete a stack is counted or a sufficient amount of signatures have been fed to complete a stack of predetermined dimension, a gapper apparatus, positioned upstream along the conveyor feed path, interrupts the flow of the sheet materials. A gap is thus formed in the stream of sheet materials. After a predetermined delay, the gapper apparatus releases the sheet materials and the stream continues. A slug of sheet materials will be present at the beginning of the next group of sheet materials to be stacked, the slug being caused by the gapper stopping the flow of sheet materials while the conveyor continued to drive the remainder of the sheet materials located upstream of the gapper. After a stack is completely formed at the stacking workstation, the stacking platform holding the completed stack descends downward to make room for a new succeeding stack and to move the completed stack to an intermediate platform. An interceptor is immediately positioned when a gap is sensed to accept sheet materials which will form the next stack. The formation of a gap is necessary to provide sufficient time for proper operation of the movable platform and operation of the interceptor. The completed stack is transferred from the platform to the intermediate platform where it is held until clamped and removed. Prior to clamping the stack, it is desirable to have the stack top as level as possible. After the previously formed stack is removed from the stacker, the platform ascends and meets the descending interceptor to take over the holding function of the new stack being formed. The interceptor is then retracted and is repositioned ready to accept the next stack to be formed after another gap is sensed.
An articulating upper belt assembly such as disclosed in a pending application of Harry Conrad Noll, Jr., entitled "Articulating Sheet Material Conveyor", U.S. Application No. 601,806, which is assigned to the assignee of this invention, can be used with mine and Mr. Chandhoke's stacker to sense the top of the sheet materials being stacked and to control the descent rate of the movable stacking platform. A device utilized to clamp and remove a completed stack of signatures from a stacker is disclosed in a pending application of Frank H. Convey, Jr., entitled "Signature Transfer Vehicle With Stack Clamping Mechanism", U.S. Application No. 518,014, now U.S. Pat. No. 4,498,381 and in a pending application of James C. Wise, entitled "Signature Handling Apparatus", U.S. Application No. 525,840, both assigned to the assignee of this invention. Signatures are typically folded when they are stacked.
A stack of signatures contains a certain amount of air as a result of folds in the signatures. Prior to clamping and removing a completed stack of signatures from the stacker, it is desirable that the stack be neatly formed with the top of the stack as level as possible by reducing the air content.
Some devices have been proposed to remove air from a stack of signatures. One such device is proposed in U.S. Pat. No. 3,617,055 to Stal. This patent proposes that the signatures be compressed by the feeding conveyors. Belts are provided to move through the stacking region at a rate faster than the movement of the buckets through the stacking region to further compress the forward folded edges of the signatures downward as each bundle is being formed.
Another device proposing to remove air from stacks of signatures is U.S. Pat. No. 4,296,684 to Wangerman. This patent proposes a stack presser to press the folded edges of the signatures downward.
These two proposed solutions apply a non-adjustable amount of force against one side of the stack. If the top of the stack is not being slanted because of fold-air build-up or is being slanted more than anticipated, the proposed devices continue to apply the same amount of downward force to the one side.